industrial electrical automation motor control technology

Tuesday, May 24, 2011

Single Phase Capacitor Start and Capacitor Run Electric Motor Control-A basic industrial process automation control technology how to do guide

While three (3) phase AC induction motors have the practical advantage for industrial applications involving large capacity motors, single (1) phase AC motors falls under the category of small size motors most suitable for smaller applications. Capacitor start and capacitor run single phase AC motors are electrically operated rotating devices most popular than its counterpart split phase AC motor due to its favorable advantage of providing high starting torque for applications where the starting load to be driven is relatively heavier than the usual load.

Capacitor start and capacitor run AC motors are mostly found in some industrial applications such as the small size motors in submersible water pumps, but as well as industrial applications are concerned, single (1) phase fractional horsepower capacitor motors are also used with household appliances such as those found in refrigerator compressors, air conditioning compressors and washing machines to name a few.

The most notable feature of the capacitor motor is its physical construction, which obviously as the name implies, comes equipped with a capacitor unit attached near the body of the motor itself. This can be distinguished by the presence of a cylindrical hump or an arched rounded container attached on the outside of the motor's case where the capacitor is housed.

Obviously enough, a capacitor start motor will only have one cylindrical hump, whereas the capacitor start and capacitor run motor will have two of such cylindrical containers protruding on the body of the motor intended to house the two capacitors.

Single Phase Capacitor Start Motor Circuit
Single Phase Capacitor Start Motor Circuit
The electrical schematic diagram on the right shows an illustration of a capacitor start motor. L1 and L2 are designated as the two connection points representing the two electricity flow path inherent with single phase circuits where a single phase supply voltage is fed to the motor's internal circuit.

Electricity is initially supplied to L1 and L2 to both the start and the run coil windings including the start capacitor during the startup run of the motor from standstill. Once the motor has gained enough rotating velocity, the start coil and the start capacitor are switched OFF by opening the contact of the centrifugal switch which is mechanically actuated by the building up of centrifugal force from the rotating velocity of the motor's rotating shaft, thereby placing the motor on its run state with only the run coil acting on the rotation of the rotor.

The centrifugal switch provides the electrical switching function with spring action that actuates the mechanical contacts which turns ON and turns OFF as a result of the driving force of the rotating speed from the center of the motor's rotating shaft.

When the motor is turned OFF by removing the power from L1 and L2, the centrifugal switch would also return to its normal close state as the motor slows down until its rotating speed gradually dies down, until finally coming to a full stop. When the centrifugal switch is returned to its normal close state the start capacitor is connected once again to the motor's circuit to prepare it for another start sequence.

Apart from the small size capacitor start motor, another variation of its type used with larger capacity motors for driving heavier loads is the capacitor start and capacitor run motor. It features the same higher starting torque capability equipped with a start capacitor but with another capacitor added to provide better performance to achieve the required capacity for the load to be driven.

Single Phase Capacitor Start and Capacitor Run Motor Circuit
Single Phase Capacitor Start and Capacitor Run Motor Circuit
The electrical schematic diagram on the right shows an illustration of a single (1) phase capacitor start and capacitor run motor. L1 and L2 are for connecting the two current path of a single phase supply voltage.

The motor initially runs with the start capacitor connected via the close contact of the centrifugal switch. Once the motor reaches sufficient running velocity, the close contact of the centrifugal switch is mechanically opened by the rotating force coming from the center of the motor's speeding shaft which comes into effect as the motor gradually picks up speed.

When the centrifugal switch is open, the start capacitor is disconnected from the supply voltage while only the run coil and the auxiliary coil and the run capacitor remains supplied with voltage.

When the motor is turned OFF by removing the power from L1 and L2, the centrifugal switch would also return to its normal close state as the motor slows down until its rotating speed gradually dies down until it finally comes to a full stop. When the centrifugal switch is returned to its normal close state the start capacitor is connected once again to the motor's circuit to prepare it for another start sequence.

Learn also about the wiring configuration for dual voltage and reversing the rotation of a single phase capacitor start motor.

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